One class of spray gun uses pressurized air for atomizing liquid such as paint and for shaping the envelope or pattern of the atomized liquid as it is discharged from a nozzle assembly on the gun. Air atomization spray guns broadly fall into two classes. One type of air atomization spray gun uses a low volume flow of high pressure air for atomization and pattern shaping. The air pressure typically may be on the order of from 40 psig (2.81 Kg/cm.sup.2) to as high as 100 psig (7.03 Kg/cm.sup.2), or more. The spray gun includes a nozzle assembly having a fluid tip and an air cap. The fluid tip has an orifice which discharges the fluid for atomization. The air cap forms an annular air discharge orifice which surrounds the fluid orifice and also has a pair of horns which discharge air from orifices for shaping the envelope of the atomized fluid into a flat, fan shaped pattern. The nozzle assembly receives the fluid and compressed air and discharges and atomizes the fluid. The fluid may be delivered to the nozzle assembly through pressure feed, gravity feed or suction feed. For many applications, the fluid is drawn through the nozzle assembly by suction produced by the flow of high pressure air. The air is discharged through an annulus which surrounds a fluid orifice, creating a suction at the orifice. The suction is sufficient to draw the fluid, such as paint, from a cup attached to the gun through supply passages in the gun to the orifice where it is discharged and atomized. For many applications, such as automotive refinishing, suction feed is preferred because of the ease of use and cleanup. The equipment is much easier to clean for color change, for example, than pressure feed equipment which requires a pressure pot and hoses connecting the pressure pot to the spray gun.
A second type of air atomization paint spray gun uses a relatively high volume flow of low pressure air for atomization and pattern shaping. The lower air pressure imparts a lower velocity to the atomized paint. The lower velocity droplets are less prone to be deflected from the surface being coated. Consequently, the transfer efficiency is increased and less paint may be dispersed into the environment. The pressure of the atomization and pattern shaping air used for HVLP spay guns is generally less than about 15 psig (1.05 Kg/cm.sup.2) and often is kept to less than 10 psig (0.703 Kg/cm.sup.2) to comply with government regulations. Some jurisdictions, for example, provide more lax air pollution control regulations if the air discharge pressure at the nozzle is no greater than 10 psig (0.703 Kg/cm.sup.2). The low air pressure may be produced either through the use of a high volume low pressure air turbine or by using a conventional high pressure compressed air source and suitable means for lowering the air pressure and increasing the volume flow, such as calibrated pressure dropping orifices or a pressure regulator.
In the past, HVLP paint spray guns have generally used pressure feed for the paint and sometimes have used gravity feed. Because of the low atomization air pressure, suction feed has not been very successful. The low air pressure has produced insufficient suction to achieve an acceptable paint flow rate. For example, one previous attempt to suction feed paint to a spray gun having a standard nozzle assembly produced a paint flow rate of only 10 to 30 grams/minute. Another attempt with existing combinations of air caps and fluid tips only achieved a flow rate of 150 grams/minute. These flow rates are inadequate for production operations, such as commercial automobile refinishing. For commercial applications, a minimum acceptable flow rate is about 200 to 220 grams/minute and a higher flow rate is preferable.
In prior art HVLP paint spray guns, the pattern of the atomized paint has not always had an optimum particle distribution uniformity. It has been found that particle distribution non uniformity for pressure feed guns is generally better than for gravity and suction feed guns and that the particle distribution uniformity is worse with suction feed.